Electrowinning processes are generally carried out in undivided electrochemical cells containing an electrolytic bath and a multiplicity of anodes and cathodes; in such processes, such as for instance copper electrodeposition, the electrochemical reaction taking place at the cathode, generally made of stainless steel, leads to the deposition of copper in metallic form on the cathode itself. At the anode, generally made of lead, as the result of the electrochemical reaction gaseous oxygen is produced, which is detached from the electrode surface in form of bubbles migrating toward the electrolyte surface. Once they reach the free surface of the electrolyte, bubbles break giving rise to an acidic mist (aerosol), fundamentally consisting of acid electrolyte droplets suspended in the atmosphere overlying the electrolytic bath. Acid mists, besides being noxious for the health of people working in the surrounding environment, are corrosive and dangerous for all metal parts of the cell room and may damage the instrumentation present.
Several chemical and physical techniques are known and used for controlling the concentration of acid mists released in the environment surrounding metal electrodeposition cells; these include the employment of surfactants and mechanical methods such as for instance the use of layers of beads floating on the electrolyte surface, which force the gas bubbles along a tortuous path where separation of acid mists takes place.
Recently there have been attempts to replace lead anodes, subject to releasing noxious material in the course of time, with non-consumable anodes obtained on a superficially catalysed substrate of titanium or other valve metal. Besides guaranteeing a better energy efficiency, this kind of anode is more resistant to corrosion also circumventing the problem of lead impurities produced during the process.
It was nevertheless observed that oxygen evolution on the latter kind of anodes evolves oxygen in form of bubbles of much reduced size (microbubbles), leading to a higher release of acid mists compared to lead anodes. The above cited methods for controlling acid mists don't have therefore the same efficacy.
It has thus been evidenced the need of providing a new system suitable for reducing or eliminating acid mists in electrodeposition processes making use of valve metal anodes comprising superficial catalytic layers.